The present invention relates to an electrochemical apparatus for reducing the alkali metal chlorate content of the liquor product of a chlor-alkali cell. The apparatus of this invention can be suitably employed in a commercial scale chlor-alkali plant utilizing a large number of chlor-alkali cells and a multi-stage caustic evaporator.
A typical electrolytic cell for the eletrolysis of aqueous alkali metal chloride solutions is an enclosed container physically partitioned into at least two distinct regions or chambers by means of an intermediate barrier or cell separator. The separator can either be a porous member, such as an asbestos diaphragm, a polymer-reinforced asbestos diaphragm, or a synthetic microporous separator, or alternatively, an impermeable ion exchange membrane. In an electrolytic diaphragm cell containing a porous separator spaced between alternating anodes and cathodes, the cathode compartment contains an electrolytic solution, i.e. catholyte, which, for a commercial scale cell, contains approximately 12%-17% of sodium hydroxide, 15%-20% of sodium chloride, and 0.5 g./l.-0.6 g./l. of sodium chlorate.
A portion of the catholyte, e.g. 6-10 gal./min. for a Hooker H-4 cell, is continuously removed from the cell as cell liquor product and purified and concentrated by the evaporation of water in multi-stage evaporation equipment.
Commercial evaporators for concentrating the cell liquor generally operate at elevated temperatures and under elevated pressure conditions utilizing steam as a heat source to produce caustic soda having a concentration of 50% or more. Such evaporators are generally fabricated of nickel or iron-based alloys such as E-brite. Under these evaporation conditions, a sodium chlorate concentration in the cell liquor feed to the evaporator of as little as 0.5 g./l. to 0.6 g./l. can severely corrode the internal evaporator surfaces and the internal surfaces of other metal components associated with the evaporation of the cell liquor. Although it has been suggested that such surfaces and components can be fabricated from titanium which is more resistant to this type of corrosion than nickel, the cost of fabricating such titanium components tends to be prohibitively high.
In addition to the corrosion problems associated with the presence of sodium chlorate in the cell liquor, sodium chlorate also represents an impurity or contaminant for certain end uses of the concentrated caustic soda, such as in the manufacture of rayon.
Various attempts have been made in the past to reduce the sodium chlorate concentration in chlor-alkali cell liquor prior to evaporation.
U.S. Pat. No. 3,380,806, issued Apr. 30, 1968, discloses a process for reducing the chlorate content of chlorate-containing solutions by adding sorbitol or glycerin to the solution and heating the solution to a temperature above about 100.degree. C. The sorbitol and/or glycerin reacts with the sodium chlorate under these conditions to produce sodium chloride, water, carbon dioxide and carbon monoxide.
Another process for reducing the chlorate content of chlorate-containing solutions is disclosed in Netherland Patent Application No. 7,603,314, published Oct. 4, 1977, wherein hydrogen is reacted with the solution under high temperature and pressure conditions in the presence of a carbon-supported ruthenium catalyst to produce sodium chloride and water as illustrated by the following reaction: ##STR1##
Although the prior art discloses various methods of reducing the chlorate content of the alkali metal chloride cell liquor product, such methods tend to rely on chemical treatment employing elevated temperature and/or pressure conditions coupled with the use of expensive catalysts or chemical systems, such as shown in equation (1). The present invention represents a departure from the prior art since an electrochemical system is utilized to achieve chlorate content reductions at least comparable to those of the prior art using less expensive materials and lower capital and operating costs.